1. Field of the Invention
The present invention relates to an image display apparatus, and more particularly to an image display apparatus for guiding the light beam containing the image information from image generating means to the eyes of the observer through the optical system thereby displaying an enlarged virtual image of the image displayed on the image generating means, and adapted for use as so-called head mounted display (hereinafter defined as HMD) to be mounted on the head of the observer.
2. Related Background Art
The head mounted display (HMD) is an apparatus for guiding the light beam, containing the image information and generated from an image displayed on the image generating means such as a liquid crystal display apparatus (LCD) or a cathode ray tube (CRT), to the pupils of the observer through the optical system thereby presenting an enlarged virtual image of the above-mentioned image, wherein the observer achieves observation by matching the exit pupil of the HMD optical system with the pupil position of the observer.
Also the HMD is preferably compact and light in weight since it is to be mounted in use on the head of the observer and, for this reason, the image generation means is often constituted by a small-sized LCD. In the HMD utilizing LCD, the light beam diverging from each of the LCD pixels shows the highest contrast in a direction which is very slightly displaced from in the normal direction to the display surface of the LCD. For this reason, the optical system of the HMD is preferably so constructed as to have the entrance pupil of the LCD side at an approximately infinite position, thereby realizing so-called telecentric system at the entrance side, and most of the proposed HMD""s have the optical system of such entrance side telecentric configuration. Also in the HMD""s utilizing small-sized CRT, the entrance side of the HMD is usually constructed as a telecentric system.
While the exit pupil of the optical system of the HMD is fixed, the pupils of the observer are movable by the movement of the visual axis. For this reason, the exit pupil of the optical system of the HMD is preferably made large, so as to be able to cover the movement of the pupils of the observer.
FIG. 2 is a schematic view showing the range of movement of the pupil of the observer, resulting from the movement of the visual axis, wherein a schematically represented eyeball 100 of the observer rotates about a center 101 of rotation when the visual axis moves. IC indicates the pupil of the observer, with a pupil diameter p, when the visual axis of the observer is directed straight ahead. The pupil IC is positioned at a distance r from the center of rotation of the eyeball. For an entire angular width w of the displayed image, the pupil of the observer moves to a position IR when the visual axis is shifted to the right-hand end, or a position IL when the visual axis is shifted to the left-hand end.
Consequently, with the movement of the visual axis within the field of view w, the pupil of the observer moves within a range represented by a line segment AB, of which length s is given by the following equation (1):
s=2xc2x7{rxc2x7sin (w/2)+(p/2)xc2x7cos (w/2)}xe2x80x83xe2x80x83(1)
The diameter of the exit pupil of the HMD optical system is preferably at least equal to the length s given by the equation (1). Consequently, the length s is called the desirable exit pupil diameter. In the equations of the present text, a markxc2x7indicates multiplication.
In general, the field of view WH of the HMD in the horizontal direction is selected larger than that WV in the vertical direction. On the other hand, the light illuminating the LCD is generally symmetrical about the axis perpendicular to the light-emitting face of the light source and, if certain means is provided, for example for increasing the numerical aperture of the illuminating light beam, in order to obtain an exit pupil diameter sufficiently large for the image display angle in the horizontal direction, the exit pupil diameter may become excessively large for the vertical direction to deteriorate the efficiency of utilization of the LCD illuminating light.
On the other hand, because of the characteristics inherent to the LCD, within the light beam outgoing from each of the liquid crystal pixels, the light beam showing a high contrast and suitable for image display is limited to a certain diverging angle (xcex1/2) with respect to the normal line to the display surface of the LCD. The entire width a will be called an effective viewfield angle of the LCD.
FIG. 7 is a view showing the relationship, in the optical system of HMD, among the effective viewfield angle xcex1, the size h of the LCD display area, the field of view w of the HMD and the exit pupil diameter q of the optical system, wherein shown are a schematically illustrated optical system 2 of the HMD, an LCD 3 and an exit pupil 3 of the HMD optical system. The light beam entering from the LCD 3 into the HMD optical system 2 is assumed to be telecentric (a ray outgoing from the display surface of the LCD 3 perpendicularly thereto being the principal ray). In this state there stands the following relation (2) wherein f is the focal length of the HMD optical system 2 and h is the width of LCD:
f=h/{2xc2x7tan (w/2)}xe2x80x83xe2x80x83(2)
As the virtual image displayed by the HMD optical system 2 is generally distanced from the pupil by as large as 1 to 2 meters, the light beams from the different points on the display surface of the LCD reach the exit pupil of the optical system in substantially parallel light beams. As shown in FIG. 7, a pair of peripheral rays R1, R2 outgoing from a point C on the LCD 3 within the diverging angle a in the plane of drawing become parallel light beams after passing the optical system 2 and pass the exit pupil plane in this state. The distance between a pair of peripheral rays R1, R2 at the position of the exit pupil represents the exit pupil diameter q in the direction of the plane of drawing paper.
Consequently the following relation (3) stands among the effective viewfield angle xcex1 of the LCD, the entire focal length f of the HMD optical system 2 and the exit pupil diameter of the optical system:
q=2xc2x7fxc2x7tan (xcex1/2)xe2x80x83xe2x80x83(3)
The relations (2) and (3) lead to the following relation among the exit pupil q, the field of view w of the HMD and the size h of the LCD display surface:
q=hxc2x7tan (xcex1/2)/tan (w/2)xe2x80x83xe2x80x83(4)
The value q represented by the equation (4) indicates the maximum exit pupil diameter formed by the light beam of high image contrast within the effective viewfield angle xcex1 of the LCD and will be called the exit pupil diameter of the optical system.
By substituting, for the distance r from the center of the eye to the pupil and the pupil diameter p, r=13 mm and p=2 mm as the average values for human being in the equation (1) and also substituting xcex1=30xc2x0 as the effective viewfield angle of ordinary small-sized LCD""s in the equation (4), there are obtained:
desirable exit pupil diameter s=2xc2x7{13xc2x7sin (w/2)+cos (w/2)}xe2x80x83xe2x80x83(1)
optical system exit pupil diameter q=hxc2x7tan (15xc2x0)/tan (w/2)xe2x80x83xe2x80x83(4)
and the comparison of the equations (1)xe2x80x2 and (4)xe2x80x2 indicates that the desirable exit pupil diameter of the HMD optical system increases with the increase of the entire angular display width w while the exit pupil diameter q of the optical system, formed by the light beam within the effective viewfield angle xcex1 of the LCD decreases with the increase of the field of view w.
Consequently, for a given size h of the LCD display surface, the field of view w becomes limited if the display is made with the light beam of high image contrast within the effective viewfield angle xcex1 of the LCD.
Also according to the equation (4)xe2x80x2, the exit pupil diameter q of the optical system is proportional to the size h of the LCD display surface. In case of HMD, the size of the LCD cannot be made very large for mounting on the head of the observer, so that the field of view w of the HMD cannot be made large.
On the other hand, a larger field of view w is a preferred factor in the virtual reality which is a representative application of the HMD, so that the limitation in the field of view w is a significant drawback.
Furthermore, as having, already explained in the foregoing, the field of view WH of the HMD in the horizontal direction is generally selected larger than that WV in the vertical direction, while the effective viewfield angle xcex1 of the LCD is generally symmetrical about the normal line to each point on the display surface of the LCD and, if certain means is provided, for example for increasing the effective viewfield angle of the LCD, in order to obtain an exit pupil diameter qH sufficiently large for the field of view WH in the horizontal direction, the exit pupil diameter qH for the vertical direction may become excessively large whereby the efficiency of utilization of the LCD illuminating light becomes deteriorated.
The object of the present invention is to provide an image display apparatus adapted for use in a head mounted display characterized in that in illuminating image generation means with an illuminating light beam from an illuminating panel light source and guiding the light beam, from the image displayed on the image generation means, to the pupils of the observer thereby causing the observer to observe a virtual image of the above-mentioned image, to convert the light beam of high intensity from the illuminating panel light source or the light beam of high contrast suitable for image display, from the image generation means, into the light beam having different diverging angles in the horizontal and vertical directions, and to realize, with such light beam, an exit pupil diameter of the optical system satisfying the desirable exit pupil diameter corresponding to the horizontal and vertical image display angles of the image display apparatus, thereby providing a bright or high-contrast image without eclipse of the pupil at the peripheral image angle even when the observer shifts the visual axis.
The above-mentioned object can be attained, according to an aspect of the present invention, by an image display apparatus comprising:
image generation means for displaying an image on a display surface constituted by a plurality of pixels;
illumination means for illuminating the image;
an optical system for guiding the light beam outgoing from the illuminated image to the pupils of the observer thereby causing the observer to observe a virtual image of the above-mentioned image formed by the light beam; and
diverging angle conversion means for setting a first direction and a second direction substantially perpendicular to the first direction in the forming plane of the virtual image, and causing the light beam outgoing from each pixel to enter the optical system with different diverging angles in the first and second directions.
The light beam having the different diverging angles in the first and second directions and entering the optical system forms, after outgoing therefrom, an exit pupil having different diameters in the first and second directions.
The diverging angle conversion means is positioned between the illumination means and the image generating means, and the light beam outgoing perpendicularly to the light-emitting surface of the illumination means and directed toward one of the pixels illuminates the image with a predetermined diverging angle formed by the diverging angle conversion means and then enters the optical system in the form of a light beam of a substantially same diverging angle.
In the light beam outgoing perpendicularly from the light-emitting surface of the illumination means and directed toward one of the pixels, two pairs of peripheral rays in the first and second directions are converted by the diverging angle conversion means into a pair of peripheral rays having different diverging angles in the first and second directions to illuminate the image and, after passing the optical system, at the exit pupil position thereof, a pair of peripheral rays in the first direction are mutually separated by a distance qH represented by:
2.4xc2x7{13xc2x7sin (WH/2)+cos (WH/2)}xe2x89xa7qHxe2x89xa72xc2x7{13xc2x7sin (WH/2)}
while a pair of peripheral rays in the second direction are mutually separated by a distance q, represented by:
2.4xc2x7{13xc2x7sin (WV/2)+cos (WV/2)}xe2x89xa7qVxe2x89xa72xc2x7{13xc2x7sin (WV/2)}
wherein WH stands for the field of view of the virtual image in the first direction, and WV stands for the field of view of the virtual image in the second direction.
The diverging angle conversion means is positioned between the image generation means and the optical system, and the light beam suitable for image display, outgoing from the pixel is subjected to the conversion of the diverging angle by the diverging angle conversion means and then enters the optical system.
In the light beam suitable image display, outgoing from the pixel, two pairs of peripheral rays in the first and second directions are converted by the diverging angle conversion means into a pair of peripheral rays having different diverging angles in the first and second directions to illuminate the image and, after passing the optical system, at the exit pupil position thereof, a pair of peripheral rays in the first direction are mutually separated by a distance qH represented by:
2.4xc2x7{13xc2x7sin (WH/2)+cos (WH/2)}xe2x89xa7qHxe2x89xa72xc2x7{13xc2x7sin (WH/2)}
while a pair of peripheral rays in the second direction are mutually separated by a distance qV represented by:
2.4xc2x7{13xc2x7sin (WV/2)+cos (WV/2)}xe2x89xa7qVxe2x89xa72xc2x7{13xc2x7sin (WV/2)}
wherein WH stands for field of view of the virtual image in the first direction, and WV stands for the field of view of the virtual image in the second direction.
The diverging angle conversion means includes an anamorphic microlens having different focal lengths in the first and second directions.
The diverging angle conversion means includes plural cylindrical microlenses having different focal lengths in the first and second directions.
The present invention will be clarified in detail by embodiments thereof to be explained in the following.